Capping machines for the application of prethreaded closures onto prethreaded containers have been known for some time. In order to insure that a prethreaded closure is not applied too tightly, which could possibly result in damage, conventional screw capping machines are provided with a screw capping head having a torque dependent clutch. The clutch limits the maximum torque which can be transmitted to the prethreaded closure and clutches of various types have been used. These have included slipping clutches and mechanical torque limiting clutches and also magnetic clutches such as those disclosed in U.S. Pat. Nos. 4,364,218, 4,492,068, 4,674,264, and 5,197,258. While these various types of magnetic clutches have been generally satisfactory, they do not adapt themselves to ready adjustability, thus resulting in substantial down time when changes in the amount of applied torque are required due to different closures being applied, application forces required, etc.
Briefly, the present invention relates to a screw capping head assembly for applying prethreaded closures onto prethreaded containers which provides for readily changing the torque to be applied to the various closures, thus making the capping head adaptable to be used with a variety of different closures and containers. A capping head incorporating the instant invention includes a first housing assembly adapted to be secured to a rotatable drive spindle and a quill mounted in an adjacent housing assembly interconnected but permitting free rotation with respect to the first housing assembly. A chuck is coaxially secured to the quill and has a tapered internal surface for gripping a prethreaded closure during the capping operation. The capping head includes a magnetic clutch comprising a first circular ring of magnets secured to the housing portion of the capping head to which a drive spindle is connected and a second coaxial spaced ring of magnets secured to the housing assembly to which the quill is connected. The gap between these axially spaced sets of magnets determines the torque to be applied to the closure. The clutch is capable of slipping after the application of a prethreaded closure onto a prethreaded container when the desired torque value has been reached. Essentially, the mode of operation is that as the chuck engages the cap upon the container to be capped, a top load is provided for cap-to-container sealing and/or for adequate rotary frictional engagement between the chuck and the cap, by slight telescoping of the spindle and housing to compress a spring disposed therebetween.
From the above description, it will be seen that the magnetic attraction between the magnetic rings will impart a torque load from the housing which rotates with the drive spindle to the chuck assembly. This torque load permits the chuck assembly to engage and tighten a cap which has previously been threadably engaged with a container to a predeterminable tightness beyond which the mechanical resistance to further tightening overcomes the magnetic attraction. When this occurs, the magnetic clutch merely slips as the spindle and associated housing continue to rotate with respect to the quill assembly.